The ultrasonic range finder is great for autonomous control of the robot. The way it works is to send an ultra sonic wave out. When that wave hits an object, the wave is reflected back. When the sensor detects that reflected signal, it will calculate the distance the sensor is from that object (using the time difference and the speed of sound in air). For the most part this is a pretty accurate device. However its not without issues. First, the sensor doesn't know what object it's looking, it just knows wether something is in front or not. So this could not really be used to track objects. A better use is in object avoidance. The other issue I've had with these is that they will sometimes pickup random signals and give crazy measurements. So you should always take about 5 measurements and average them to get a more accurate reading.
